Heart rate monitors are used for measurement of a heart rate, e.g., for applications such as health monitoring, sports training, and fitness. Conventional devices capable of constantly monitoring a user's heart rate have been, until recently, cumbersome. For example, heart rate monitors often use electrocardiogram (EKG) signals, which require electrodes or a strap placed around the user's chest.
A less cumbersome technique for monitoring heart rate is based on pulse oximetry using a photoplethysmograph (PPG) sensor. A PPG sensor non-invasively measures the absorption of light passing through a user's tissue, e.g., a finger or an ear lobe, to determine the oxygen saturation level of arterial blood and heart rate. Arteries expand and contract due to blood flow and, thus, the amount of absorbed light changes in the course of a heart beat. The resulting signal is referred to as a photoplethysmograph (PPG). The PPG signal may be analyzed to determine, among other thing, the heart rate of the person to which the PPG sensor is attached.
Conventionally, pulse oximetry requires the user to remain relatively motionless to ensure that a good PPG signal is received. When a PPG sensor is used with a person that is in motion, artifacts in the PPG signal may be produced as a result of the user's motion as well as displacement of the PPG sensor. The resulting artifacts or noise result often result in inaccuracies in the estimated heart rate of the user.